The invention relates to a process for production of cross-linked polymer moldings of thermoplastic base polymers in pourable form which are mixed with pourable carrier polymers dispersable or soluble in the basic polymer with a cross-linking agent, these mutually compatible components are heated under further mixing to a reaction temperature above the melting range of the basic polymer where the reaction of free radicals creates bonds between polymer chains and/or breaks down polymer chains.
Polymers consist of molecular chains with numerous components which repeat each other practically endlessly, which differ from low molecular compounds in relation to physical properties, for example in relation to their high tensile strength and elasticity. As a result of cross-linking of polymer chains, many material properties are changed. Active substances are produced which are superior to the original materials in essential features. Almost all natural and synthetic thermoplastics can be cross-linked. The term thermoplastics or thermoplastic polymers here and below also includes elastomers, in particular thermoplastic elastomers.
Free radicals have one or more unpaired electrons, for example the methyl radical H3C* or the triphenylmethyl radical (C6H5)3C* which has been known for some time. Radical reactions are chain reactions which are instigated by small quantities of reactive free radicals and maintained as chain reactions. Organic free radicals are usually produced by heating compounds with weak covalent bonds. Thus organic peroxides which can easily be split homolytically at the weak O--O bond are initiators for radical reactions. Numerous organic molecules can be split by visible or ultraviolet light, under formation of free radicals. Often a more energy-rich radiation is used, for example .beta. radiation also known as electron radiation, or .gamma. radiation.
In contrast to polymerization, cross-linking is more stoichiometric and leads to a cross-linking yield.
The free radicals formed with a cross-linking agent or by radiation can extract a hydrogen atom from the polymer and become saturated with this. Thus a polymer radical is produced with an unpaired electron. Two of these polymer radicals can link together forming a new C--C bond. Multiple repeats of this reaction create a three-dimensional network from the originally linear polymer chains.
The structure of the basic polymers has an effect on the cross-linking. In addition to the coupling reaction specified above, polymer chains can also be split. In other words, several polymers under the effect of radicals do not link with other chains but are broken down and thus cross-linked. This applies in particular to heavily branched components with many tertiary bonded C-atoms in the main chain. For example, polypropylene (PP), polybutylene (PB), and polyvinylchloride (PVC) are not cross-linked but are broken down. This breaking down, also called degradation, is covered by the present invention with the term cross-linked or cross-linking.
The production of cross-linked polymers using silanes, as is described for example in EP,A2 0426073, is not included in the task of the present invention. According to EP,A2 0426073, a cross-linkable polymer is exposed to moisture in a targeted manner in the presence of a hydrolysis condensation catalyst. The process comprises the mixing of a thermoplastic basic polymer with an essentially water-free solid carrier polymer which contains a silane and a cross-linking agent generating free radicals. This mixture is heated to a temperature above the melting point of the basic polymer and mixed further.
U.S. Pat. No. 4,247,498 describes microporous carrier polymers and a process for their production. The microporous carrier material can absorb relatively large quantities of "functionally useful fluids" and remain a solid body in nature. The microporous products are produced from synthetic thermoplastic polymers and fluids compatible with these. During the production process, some of the compatible fluids are removed again. The compatible fluids may also be the functionally useful fluids mentioned above if these allow direct use. U.S. Pat. No. 4,247,498 does not mention any such cross-linking agents introduced into the pores for polymers.
U.S. Pat. No. 4,783,511 describes a process for the production of cast objects of dry thermoplastic polyamides which have improved mechanical properties and greater dimensional stability than the original polyamide materials. The process includes the addition of dry polyamide "master batch", which comprises a silane and a substrate, for example a polyolefin, a swelling material or a non-swelling thermoplastic sponge. Three processes for the production of a "master batch" are described.